The present invention relates to systems and methods for non-destructive quality control in general and to optical systems and methods for measuring the thickness and an index of refraction of thin films, in particular.
Optical measuring instruments are typically utilized in the microelectronic industry for non-contact, non destructive measurement of the thickness of thin films. Two main systems are utilized, spectrophotometers (or reflectometers) and ellipsometers. The following U.S. patents represent the prior art:
For ellipsometers: U.S. Pat. Nos. 5,166,752, 5,061,072, 5,042,951, 4,957,368, 4,681,450, 4,653,924, 4,647,207 and 4,516,855.
For spectrophotometers: U.S. Pat. Nos. 5,181.080, 5,159,412, 5,125,966, 4,999,014 and 4,585,348.
The two prior art systems are illustrated in FIGS. 1A and 1B, respectively, to which reference is now made. The spectrophotometer utilizes the fact that light beams reflected off thin film boundaries, will interfere one with another. Specifically, the spectrophotometer of FIG. 1A measures the reflectance of selected points of a sample 10 as a function of the light wavelength, usually in the visible or near UV spectral ranges. Computer analysis of the detected spectral reflection function, especially its minima and maxima, provides the thickness, and in some cases, also the index of refraction of the measured film.
The spectrophotometer typically includes a transmitter 12 with a light source and appropriate optics, a beam splitter 14, an objective lens 16, a tube lens 18 and a receiver 20 which includes optical and electronic means for measurement of light intensity as a function of the input light wavelength. The transmitter 12 produces a collimated light beam 22 which is deflected by the beam splitter 14 and focused on the sample 10 by the objective lens 16. The reflected beam, labeled 24, is collected by the microscope imaging optics (lenses 16 and 18) on to a spectroscopic measurement unit within the receiver 20.
In order to measure a multiplicity of points on the sample 10, sample 10 is placed on an x-y stage 26. X-Y stage 26 is typically very precise and heavy and, as a result, moves very slowly.
The spectrophotometers have difficulty measuring structures with very small reflectance, such as thin films of glass substrates, because the relatively low brightness of traditional white light sources does not provide a sufficient signal-to-noise ration (SNR). Spectrophotometers also have difficulty measuring films with unknown or unrepeatable dispersions of optical constants, such as amorphous silicon.
Despite these limitations, the spectral photometry method is at present widely used in industry because the instrumentation for this method is easily combined with optical microscopes and can utilize conventional microscope optics.
Ellipsometers measure changes in the polarization of light caused by reflectance from the test surface. These changes, characterized as amplitude and phase changes, are very sensitive to the thickness and optical properties of thin films.
A prior art ellipsometer is illustrated in FIG. 1B. It includes a transmitter 30 which includes a light source and appropriate optics, a polarizer 32, an optional compensator (phase retarder) 34, an analyzer 36 and a receiver 38 with a photo-detector and appropriate electronics. The polarizer 32 polarizes the light beam 40 produced by light source 30. The reflected light beam, labeled 42, passes through the analyzer 36 before reaching the receiver 38. If the compensator 34 is used, it may be located either between the polarizer 32 and the test sample 10 or between the sample 10 and the analyzer 36.
The ellipsometric method requires oblique illumination, i.e. an angle of incidence "THgr" between an incident light beam 40 and a normal 44 to the sample 10 must be greater than zero. The angle between a reflected light beam 42 and the normal 44 is equal to the angle of incidence "THgr". The angle of incidence "THgr" should be close to the Brewster angle "THgr"B of the substrate. In practice, the angle of incidence "THgr" ranges from 45xc2x0 to 70xc2x0.
Because ellipsometers measure two polarization parameters (amplitude and phase), both of which are independent of the light intensity, they are quite accurate and can also measure ultra thin films of the size of 0-100 xc3x85. However, since ellipsometers require oblique illumination as well as a highly collimated light beam, their use for high spatial resolution measurements in dense patterned structures is rather difficult.
There are two basic types of fully automated ellipsometers. Null-ellipsometers (NE) provide the most accurate thickness measurements but they require at least several seconds per measuring point. Rotating-analyzer ellipsometers (RAE) provide very high speed measurements (portions of a second per measuring point), but their sensitivity and accuracy are usually less than those of null ellipsometers.
For all of the prior art instruments, the opto-mechanical apparatus is complicated, large and heavy, and thus, the x-y stage 26 is translated between measurement points, coming to a complete stop before measurement begins. The time between measurements depends on the mass of the x-y stage 26 and on the positioning accuracy requirements and may take at least several seconds (sometimes up to several tens of seconds). This limits the speed with which a thickness mapping can occur, especially during inspection of large size substrates such as 8xcex94 VLSI silicon wafers, 18xe2x80x3xc3x9718xe2x80x3 LCD glass panels, etc. 
The footprint, or space on the floor which each machine utilizes, is typically at least twice the size of the x-y stage 26 due to its translation.
Furthermore, the prior art measuring devices are utilized for measuring once a deposition process has been completed. They cannot be utilized for in-process control, since wafer handling and other mechanical movements are not allowed within a vacuum chamber.
Other measuring instruments are also known, one of which is described in U.S. Pat. No. 4,826,321. The ""321 patent presents a system similar to an ellipsometer. However, in this system, a mirror is utilized to direct a plane polarized laser beam to the thin film surface at the exact Brewster angle of the substrate on which the thin film lies.
There is provided, in accordance with a preferred embodiment of the present invention, a two-dimensional beam deflector for a thickness measuring device for measuring the thickness of films on a sample with a plurality of different optical systems each performing a different measurement technique. The beam deflector includes a two dimensional translation unit, first and second deflection units and a plurality of optical assemblies. The two-dimensional translation unit translates the beam deflector along a first scanning axis and along a second scanning axis perpendicular to the first scanning axis. The first deflection unit receives a plurality of parallel input beams along parallel input axes which are close to one another and parallel to the first scanning axis. The first deflection unit also deflects the input beams along a plurality of parallel second axes close to each other and parallel to the second scanning axis. The second deflection unit receives a plurality of parallel output beams along parallel third axes close to each other and parallel to the second axes, and deflects the output beams along a plurality of parallel fourth axes close to each other and parallel to the first scanning axis. There is one optical assembly per input beam, each of which provides its input beam towards the sample, receives its output beam from the sample, processes its input and output beams in accordance with its measurement technique, and provides its output beams along the parallel third axes.
Additionally, in accordance with a preferred embodiment of the present invention, the optical assemblies can be at least an ellipsometric assembly and a spectrophotometric assembly.
There is also provided, in accordance with a preferred embodiment of the present invention, a thickness measuring device for measuring the thickness of thin films on a sample. The device includes first and second stationary illuminators, a beam deflector and first and second stationary receivers. The first and second stationary illuminators provide first and second collimated input light beams along first and second parallel input axes. The beam deflector directs the first and second input light beams towards the sample and direct and collimate corresponding first and second output light beams from the sample. The beam deflector includes a two-dimensional translation unit for translating the beam deflector along a first scanning axis parallel to the input axis, and along a second scanning axis perpendicular to the first scanning axis. The first and second stationary receivers respectively receive the first and second collimated output light beams along output axes parallel to the input axes.
There is further provided, in accordance with a preferred embodiment of the present invention, an ellipsometer which includes a stationary illuminator, a translatable beam deflector and a stationary receiver. The stationary illuminator provides a collimated input light beam along an input axis. The beam deflector, translatable at least along a first scanning axis parallel to the input axis, includes a) a first beam deflecting element for deflecting the input light beam at an angle of deflection towards the sample, b) a second beam deflecting element, different from the first beam deflecting element, for deflecting an output light beam reflected at an angle from the sample along the output axis and c) a collimating lens for receiving at least the output light beam from the second beam deflecting element and for collimating at least the output light beam. The stationary receiver receives the collimated output light beam along an output axis parallel to the input axis.
Additionally, in accordance with a preferred embodiment of the present invention, the beam deflector comprises one-dimensional translation unit for translation along the scanning axis. Alternatively, the beam deflector comprises two-dimensional translation unit for translating the beam deflector along the first scanning axis and along a second scanning axis perpendicular to the first scanning axis.
Moreover, in accordance with a preferred embodiment of the present invention, the beam deflector additionally comprises a first mirror for deflecting the input light beam from the input axis to the second scanning axis, a second mirror for deflecting the input light beam from the second scanning axis to the sample, a third mirror deflecting a reflected light beam from the sample to the second scanning axis, and a fourth mirror for deflecting the reflected light beam from the second scanning axis to the output axis.
Furthermore, in accordance with a preferred embodiment of the present invention, the first and second beam deflecting elements are mirrors.
Additionally, in accordance with a preferred embodiment of the present invention, the first beam deflecting element is a beam splitter and the second beam deflecting element is a mirror.
Finally, in accordance with a preferred embodiment of the present invention, the ellipsometer includes a unit for measuring an actual angle of incidence which may vary from the angle of deflection, wherein the unit for measuring utilizes optical elements forming part of the stationary illuminator and stationary receiver. The unit for measuring includes a position sensing device for measuring the angle of the output light beam with respect to a desired position.